As one type of secondary cells featuring high energy density, a non-aqueous electrolyte secondary cell has come into practical use, wherein a non-aqueous electrolyte solution is employed and charge/discharge is performed by way of transferring lithium ions between a positive electrode and a negative electrode.
Such a non-aqueous electrolyte secondary cell generally employs a positive electrode which is a lithium-transition metal compound oxide such as LiCoO2 and the like, a negative electrode which is lithium metal, a lithium alloy, or a carbon material capable of absorbing and desorbing lithium, and a non-aqueous electrolyte solution wherein an electrolyte of lithium salt, such as LiBF4 or LiPF6, is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate.
Recently, further, such a non-aqueous electrolyte secondary cell has come into practical use as an electric current source of portable equipment and the like, and accordingly, the non-aqueous electrolyte secondary cell having higher energy density has been desired.
Unfortunately, however, in such an ordinary non-aqueous electrolyte secondary cell, the lithium-transition metal compound oxide such as LiCoO2 employed as the positive electrode thereof is large in weight and small in reactive electron number, therefore, capacity per unit weight is not sufficiently improved.
Further, sulfur is generally known as positive electrode material having large theoretical capacity, however, where a simple substance of sulfur is employed as the positive electrode, very high temperature is required for reversible reaction with lithium, therefore, the resultant non-aqueous electrolyte secondary cell can not come into a generous use.
Therefore, recently, there has been proposed use of an organic disulfide compound including DMcT (2,5-dimercapto-1,3,4-thiadiazole) as the positive electrode material having large capacity and high energy density. However, the organic disulfide compound used as the positive electrode material reversibly reacts with lithium only at a high temperature of more than 60° C., therefore, the resultant non-aqueous electrolyte secondary cell can not come into the generous use.
Further, more recently, Japanese Patent Application Nos.4-267073, 8-115724, and so on have proposed the use of the positive electrode material which is a compound of the organic disulfide compound including DMcT with conductive macromolecule including polyaniline for the purpose of charge/discharge reaction at normal temperature.
However, even in the use of the organic disulfide compound as the positive electrode material, a part concerned with the charge/discharge reaction is only disulfide linkage portion, and other portions such as carbon portion or hydrogen portion are not concerned with the reaction, therefore, the capacity per unit weight is not sufficiently improved.